The present invention relates generally to an optical moisture sensor for detecting moisture on the surface of a transparent material, and more particularly, to a moisture sensor autobalance circuit for preventing saturation of the sensor amplifiers.
The accumulation of moisture on transparent materials, such as glass or Plexiglass, can obstruct a person's view through the material. Motor vehicles have long been equipped with motor-driven windshield wipers for clearing the moisture from the external surface of the windshield, at least within the driver's field of vision, and generally over a larger area so as to enhance one's vision through the windshield.
In most vehicles today, the windshield wiper system includes multi-position or variable speed switches which allow the driver to select a wide, if not an infinitely variable, range of speeds to suit conditions. Wiper controls are manually operated and typically include a delay feature whereby the wipers operate intermittently at selected time delay intervals.
Wiper control systems have recently been developed which include a moisture sensor mounted on one of the vehicle windows to automatically activate the wiper motor when moisture is deposited upon the surface of the window. The wiper control system including the moisture sensor are most typically mounted on the windshield, although the system may be mounted on the rear window or any other glass surface intended to be cleared of moisture. Such wiper control systems free the driver from the inconvenience of frequently adjusting the wiper speed as the driving conditions change.
Wiper control systems have used a number of different technologies to sense the moisture conditions encountered by a vehicle, including conductive, capacitive, piezoelectric, and optical sensors. Optical sensors operate upon the principle that a light beam is diffused or deflected from its normal path by the presence of moisture on the exterior surface of the windshield. The systems which employ optical sensors have the singular advantage that the means of sensing disturbances in an optical path is directly related to the phenomena observed by the driver (i.e., disturbances in the optical path that affords the driver vision). McCumber et al. (U.S. Pat. No. 4,620,141) disclose an optical moisture sensor which triggers a sweep of the wiper blades in response to the presence of water droplets on the exterior surface of a windshield.
In optical moisture sensors, a light signal from an emitter is directed into the windshield and reflected back by the outer surface of the windshield and into a detector. The presence of moisture on the surface of the windshield affects the reflection of the light signal at the outer surface of the windshield resulting in a reflected signal having a lower amplitude. The detector receives the reflected signal and produces an output signal which indicates the change in amplitude of the reflected emitter signal. The detector output signal also contains noise and other unwanted signals such as those from ambient light and electromagnetic interference. These undesirable signal components must be removed from the detector output signals before accurate moisture readings can be obtained.
It is known to use high pass and low pass filters to remove unwanted signal components from the detector signals. Noack, in U.S. Pat. No. 4,355,271, shows a moisturesensor with a detector having an output connected to a filter. The filter rejects the low frequency components of the signal from the detector, and provides gain to the signal. The signal is subsequently demodulated using a rectifier, and compared against a threshold. The filter of '271 is thus a pre-demodulation filter which acts on the signal before a demodulator converts the pulsatile signal into a dc signal that is affected by sensed moisture.
However, the use of pre-demodulation gain tends to make the gain stage circuitry prone to saturation, in which the output of the amplifier reaches the upper or lower limits of the sensor power supply. For example, large moisture drops tend to cause large changes in the signal emanating from the detectors. If a signal arising from a large drop should cause the gain stage output to reach either of those limits, the output can go no further and saturates. When operating under these conditions, the moisture sensor system becomes insensitive to further changes in the detector output and loses the ability to accurately detect moisture. It is desirable to prevent saturation to keep the sensor operating under similar conditions.
The tendency of the gain stage circuitry to saturate limits how much gain the circuitry may provide. This, in turn, limits the ability of the moisture sensor to detect small droplets of moisture. Small droplets of moisture lead to very small changes in the signal from the detectors. If the gain of the pre-demodulation circuitry is kept low enough so that large drops do not cause the pre-demodulation circuitry to saturate, then the signal arising from small drops will be too small to be detected by subsequent processing. Thus it is desirable to prevent saturation, so that the gain of the system may be sufficient to detect small drops.